1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to an apparatus and method for transmitting data using transmission diversity in a wireless communication system.
2. Discussion of the Related Art
Recently, a demand for wireless data service is abruptly increasing. Evolution from wireless voice service towards wireless data service requires a gradual increase of the wireless capacity. Such requirement enables wireless service providers and wireless equipment manufacturers to try to improve the data transmission rate of wireless systems and gives them a motive to do active research.
A wireless channel experiences several problems, such as path loss, shadowing, fading, noise, a limited bandwidth, a limit power of a terminal, and interference between users. Such a limit makes the wireless channel have a form similar to a narrow pipe which hinders the fast flow of data and also makes it difficult to design an efficient bandwidth of wireless communication which provides high-speed data transmission. Other challenges in the design of a wireless system include resource allocation, mobility issues related to a rapidly changing physical channel, portability, and the design of providing security and privacy.
If an additional version or replica of a transmitted signal is not received when a transmission channel experiences deep fading, it makes it difficult for a receiver to determine the transmitted signal. Resources corresponding to the additional version or replica are called diversity. The diversity is one of the most important factors which contribute to reliable transmission over wireless channels. If the diversity is employed, the capacity or reliability of data transmission can be maximized. A system implementing diversity using multiple transmission antennas and multiple reception antennas is referred to as Multiple Input Multiple Output (MIMO), and the MIMO system is also called a multiple-antenna system.
In the MIMO system, schemes for implementing diversity include Space Frequency Block Code (SFBC), Space Time Block Code (STBC), Cyclic Delay Diversity (CDD), Frequency Switched Transmit Diversity (FSTD), Time Switched Transmit Diversity (TSTD), Precoding Vector Switching (PVS), Spatial Multiplexing (SM), Generalized Cyclic Delay Diversity (GCDD), and Selective Virtual Antenna Permutation (S-VAP) and the like.
Meanwhile, one of systems taken into consideration in systems after the third generation is an Orthogonal Frequency Division Multiplexing (OFDM) system capable of attenuating the inter-symbol interference effect through low complexity. In the OFDM system, serial input data is converted into an N number of parallel data, carried on an N number of orthogonal subcarriers, and then transmitted. The subcarriers maintain orthogonality in the frequency domain. Orthogonal Frequency Division Multiple Access (OFDMA) refers to a multiple-access method of realizing multiple-access by independently providing some of available subcarriers to each user in a system using the OFDM method as a modulation method.
However, one of the major problems of the OFDM/OFDMA systems is that the Peak-to-Average Power Ratio (PAPR) may be very high. The PAPR problem is that the peak amplitude of a transmission signal is very greater than the average amplitude. The PAPR problem is caused by the fact that an OFDM symbol is the overlapping of an N number of sinusoidal signals on different subcarriers. The PAPR is related to the capacity of the battery and problematic in a terminal which is sensitive to power consumption. In order to reduce power consumption, the PAPR needs to be lowered.
One of systems proposed to lower the PAPR is a Single Carrier-Frequency Division Multiple Access (SC-FDMA) system. SC-FDMA is of a form in which a Frequency Division Multiple Access (FDMA) method is grafted onto a Single Carrier-Frequency Division Equalization (SC-FDE) method. The SC-FDMA method has a similar characteristic to the OFDMA method in that data is modulated and demodulated in the time domain and the frequency domain, but is advantageous in terms of low transmission power because Discrete Fourier Transform (DFT) is used and so the PAPR of a transmission signal is low. In particular, it can be said that the SC-FDMA method is advantageous in uplink communication in which a terminal sensitive to the transmission power in relation to the use of the battery performs communication to a base station. An important point when a terminal sends data to a base station is that the bandwidth of transmitted data is not great, but coverage in which power can be concentrated must be wide. An SC-FDMA system has a wider coverage than other systems when the same power amplifier is used because a variation in the signal is small. Meanwhile, in a clustered DFT-S-OFDM method unlike the SC-FDMA method, M(<N) symbol strings from among DFT-Spread (S) N symbol strings are allocated (or mapped) to contiguous subcarriers, and the remaining N-M symbol strings are allocated (or mapped) to contiguous subcarriers spaced apart from one another, from among subcarriers to which the M symbol strings have been allocated (or mapped). The clustered DFT-S-OFDM method is advantageous in that frequency selective scheduling can be performed.
In using the SC-FDMA method, however, attention must be paid to the satisfaction of a single carrier property. A wireless communication system must be able to provide transmission diversity to lower the PAPR by employing the SC-FDMA method or the clustered DFT-S-OFDM method. STBC (that is, one of the above transmission diversity schemes) is a scheme for obtaining a diversity gain by using selectivity in the space domain and the time domain. There is a need for an apparatus and method for transmitting data, in which the STBC scheme is used, but transmission diversity to lower the PAPR can be provided.